Refactor: Extract commands from monolithic main.go

[dapi]

Problem

The cmd/port-selector/main.go file has grown to 625 lines and handles multiple responsibilities:

• Argument parsing
• Port selection business logic
• Output formatting
• All command implementations (list, scan, forget, lock/unlock)

This violates the Single Responsibility Principle and makes the code harder to maintain and test.

Proposed Solution

Extract commands into a separate package:

internal/
└── commands/
    ├── commands.go  # Common types and interfaces
    ├── run.go       # Main port selection logic
    ├── list.go      # --list command
    ├── scan.go      # --scan command
    ├── forget.go    # --forget, --forget-all commands
    └── lock.go      # --lock, --unlock commands

Benefits

• Better testability (each command can be tested independently)
• Cleaner main.go (just argument routing)
• Easier to add new commands
• Better code organization

Acceptance Criteria

• Create internal/commands/ package
• Extract each command into separate file
• Keep main.go under 150 lines
• All existing tests pass
• Add unit tests for extracted commands